1. Field of the Invention
The present invention relates generally to optical systems for use in photographic printers and, more particularly, to scanning optical systems using radiation devices for use with photographic printers having spatial light modulators to modulate monochromatic light.
2. Description Of The Prior Art
There are numerous types of photographic printers available that range in cost and quality. One of the more common photographic printers uses an area cathode-ray tube (CRT) to image a photosensitive medium. In this printer, a CRT projects a two-dimensional image through a lens system onto the photosensitive medium. A problem with this printer is its size. The CRT itself is large. The lens system then directs the light through a focal cone that takes a large volume. Due to these factors, the printer becomes relatively large.
Another commercially available printer uses a linear CRT with red, green and blue (RGB) phosphor stripes and a fiber optic face plate to project an image onto the photosensitive medium as the photosensitive medium moves in a direction transverse to the linear CRT. While some of the size issues are addressed by this printer, the CRT is very expensive, making the printer very costly.
A third type of optical printer uses light emitting diodes (LEDs) and lenses to project image-bearing light to an oscillating mirror which directs the image-bearing light in a fast scan direction. Motion of the photosensitive medium relative to mirror is used for slow scan direction. This printer is compact but the optics are expensive.
Accordingly, it is an object of this invention to provide an optical system for a photographic printer that is compact.
It is another object of this invention to provide an optical system for a photographic printer that is inexpensive.
The present invention relates generally to optical systems suitable for use in the conversion between a line image of uniform height and an essentially localized, relatively small image, and in particular to line printing or scanning optical systems. The optical systems disclosed comprise a concave mirror, at least one cylindrical optical element, an optional aperture stop, and an optional spatial radiation modulator. In printing and other such illumination applications, the disclosed optical systems serves to produce a line image, of uniform height and intensity, from a radiation source of a relatively small size. For example, the optical systems can be adapted for use in photographic printers which print consecutive lines of pixels of an image onto a photosensitive medium.
The cylindrical optical element, which can be a cylindrical lens or a cylindrical mirror, is positioned in optical alignment with a substantially collimated beam of radiation produced by the concave mirror, and serves to converge or focus the substantially collimated radiation along at least one axis of the substantially collimated beam.
In one implementation the light is substantially collimated in one direction by the concave mirror which in this implementation is a cylindrical optical element, and as such has a singly-curved surface. In another implementation the light is substantially collimated in two directions by the concave mirror which has a doubly-curved surface, and a source optical element adapted to partially collimate the light prior to impinging upon the concave mirror, where the source optical element can be a separate element or combined with the cylindrical optical element. In either implementation the collimating means also minimizes drop-off of a height of the line image at the extremes.
A spatial radiation modulator may be disposed along the optical path of the optical system. The spatial radiation modulator may comprise a plurality of cells, each of which is electronically selectable between radiation-blocking and radiation-transmitting states. Each cell corresponds to a pixel which is thus selectable between the two states, to form an image on the photosensitive medium by selectively transmitting the incident focused radiation. Thus, when relative movement occurs between the photosensitive medium and the optical system, consecutive imaging lines are projected onto the photosensitive medium to create an image.